1. Field of the Invention
The present invention relates generally to driver information systems and more specifically to systems which inform a driver about the propulsive capability of a vehicle powertrain.
2. Description of the Related Art
In an electric vehicle, the powertrain of the vehicle comprises one or more electric motors (called "traction motors"), which provide propulsion for the vehicle. Energy for the traction motors is stored in one or more batteries on board the vehicle known as "traction batteries". In a typical electric vehicle, the traction motors are three-phase AC induction motors. The DC power in the traction batteries is converted to three-phase AC by an inverter, a device containing power electronics.
An inherent feature of traction motors (and electric motors generally) is that they operate in a "constant torque" region at low speeds and a "constant power" region at higher speeds. In the constant torque region, the torque of the traction motor (and therefore the acceleration of an electric vehicle) is predominantly a function of the current which can be supplied by the power electronics. Low-speed torque is therefore largely independent of the state of charge of the traction batteries. The driver of the vehicle can therefore drive at low speeds (in parking lots, on side streets, and the like) without any suspicion that his traction batteries have a low state of charge.
However, a low battery state of charge very much limits the power of an electric vehicle at higher speeds. Operating at higher speeds, in the "constant power" region of the traction motor, requires power (as opposed to simply current) from the battery. Power output capability of a battery is a direct function of its state of charge.
The troublesome result of a traction motor having the "constant torque" and "constant power" regions is that an electric vehicle can feel like it has full propulsive capability while driven at low speeds, when in fact the vehicle's ability to operate at high speeds is severely limited by traction batteries with a low state of charge. Therefore, a driver may enter a high-speed driving situation (such as a freeway) without the ability to attain the high speeds required.
One way to provide information to the driver regarding the reduced propulsive capability of the traction motors is through a meter on the vehicle dashboard which indicates the state of charge of the traction batteries. However, a more intuitive means of informing the driver of the reduced capability of the traction motors can provide the driver with more useful information.
Therefore, alternative means for informing a driver of reduced propulsive capabilities of the traction motors in his vehicle can provide advantages over the prior art.